* Factor out metadata fields from Data.Conversation

* Restructure MLS fields in Conversation record

* Move protocol field from metadata to Conversation

* Refactor group conversation creation

 - Moved conversation name size check to `NewConv`
 - Made the `NewConversation` record (used as input to the data
   function creating a conversation) contain a `ConversationMetadata`.

This reduces duplication, and makes it easier for other conversation
creation functions to be implemented on top of `createConversation`.

* Consolidate conversation creation functions

* Remove conversation name limit tests

The name limits are now encoded in the types, so it is not so easy
anymore to test requests with name length exceeding the limit. It is
also arguably unnecessary.

* Add CHANGELOG entry